Computing and networking technology have transformed our world. As the amount of information communicated over networks has increased, high speed transmission has become ever more critical. Many high speed data transmission networks rely on optical transceivers and similar devices for facilitating transmission and reception of digital data embodied in the form of optical signals over optical fibers. Optical networks are thus found in a wide variety of high speed applications ranging from as modest as a small Local Area Network (LAN) to as grandiose as the backbone of the Internet.
Typically, data transmission in such networks is implemented by way of an optical transmitter (also referred to as an electro-optic transducer), such as a laser or Light Emitting Diode (LED). The electro-optic transducer emits light when current is passed there through, the intensity of the emitted light being a function of the current magnitude through the transducer. Data reception is generally implemented by way of an optical receiver (also referred to as an optoelectronic transducer), an example of which is a photodiode. The optoelectronic transducer receives light and generates a current, the magnitude of the generated current being a function of the intensity of the received light.
Various other components are also employed by the optical transceiver to aid in the control of the optical transmit and receive components, as well as the processing of various data and other signals. For example, such optical transceivers typically include an electro-optic transducer driver (e.g., referred to as a “laser driver” when used to drive a laser signal) configured to control the operation of the optical transmitter in response to various control inputs. The optical transceiver also generally includes an amplifier (e.g., often referred to as a “post-amplifier”) configured to perform various operations with respect to certain parameters of a data signal received by the optical receiver. A controller circuit (hereinafter referred to as the “controller”) controls the operation of the laser driver and post-amplifier.
During the operation of the optical transceiver, it is often important to evaluate the quality of a transmitted data signal. One tool often used to help in the evaluation process is an eye diagram or pattern. As is well known, an eye diagram is a graph illustrating, in one example, power output as a result of AC modulation. For example, a constant AC signal, such as a digital square wave at a given frequency, is used to modulate a laser. In one example, high values of the digital square wave correspond to logical 1s, while low values of the digital square wave correspond to logical 0s. The power output of the laser diode is then graphed for an integer multiple of a cycle as a function of time. Successive integer multiple of the cycle of the power output are graphed and overlaid on one another. This process provides a visual depiction of the area in which one could expect to find a high (logical 1) or low (logical 0) power output. The eye diagram can be used to quantify characteristics such as rise time, fall time, jitter, and overshoot.
As mentioned, the eye diagram may be used to measure the rise and fall time of the signal. In an ideal case, the rise and fall time of the measured signal are equal. In other words, the signal transitions form high to low or from low to high at the same rate. However, due to inherent non-linearity in most lasers, the rise time as measured on the eye diagram is faster than the fall time or the fall time is faster than the rise time. This asymmetric rise/fall time often causes signal distortion to occur.
For example, if the optical signal transitions too quickly from high to low, the optical signal may undershoot the low optical intensity used to represent the logical zero (hereinafter also referred to as the “baseline low optical intensity”). This increases the amount of time needed for the optical signal to settle to the baseline low optical intensity. If the optical intensity undershoots too far, the laser may even turn off thereby significantly increasing the settling time. If the next transition from low to high is within this settling time, the optical intensity may be above or below the baseline low optical intensity. This means that next transition from low to high may occur sooner or later than desired. Accordingly, jitter is introduced into the optical signal sequence.
If the optical signal transitions too quickly from low to high, the optical signal may overshoot the high optical intensity used to represent the logical one (hereinafter also referred to as the “baseline high optical intensity”). Once again, settling time is increased thereby introducing the potential for jitter.
The overshoot and undershoot problems discussed above may also have other undesirable effects. For example, before the electrical signal is converted into the optical signal, the existence of overshoot and undershoot may cause the emission of electromagnetic interference, thereby potentially adversely affecting the performance of the telecommunications system as a whole.
The eye diagram may also be used to observe the duty cycle or cross-point of the output signal. The cross-point is the point on the eye diagram where the transitions from high to low and low to high intersect. For example, the digital low is often represented by a 0 volt signal and the digital high is represented by a 1 volt signal. Accordingly, in an ideal system, the cross-point would be observed at 0.5 volts. This would mean that the duty cycle of the output signal was 50%, i.e., 50% of the time the signal was in a high state above the 50% percent cross-point and 50% of the time the signal was in a low state below the 50% cross-point.
However, due to the asymmetric rise/fall time discussed previously and other factors, the output signal often does not exhibit a 50% duty cycle. For example, if the rise time is faster than the slow time, the transition from low to high may occur prior to the 50% cross-point, thus having the signal remain in a high state for more than 50% of the signal cycle. When the fall time is faster than the rise time, the transition from low to high may occur prior to the 50% cross-point, thus having the signal remain in a low state for more than 50% of the signal cycle. Such duty cycle distortion often leads to signal distortion.
Accordingly, it would be advantageous to have mechanisms to compensate for asymmetric rise/fall time and duty cycle distortion.